Server virtualization involves simulating several virtual machines (VMs), each running a separate operating system (OS) instance on a single physical server. Each OS may run in a different VM. For example, Xen is a virtual machine monitor (VMM), also known as a hypervisor, for managing the VMs that execute on a host operating system to provide the functionality of several guest operating systems on top of the host, on the same computer hardware (physical machine), at the same time. Server virtualization technologies are becoming increasingly commonplace in data centers. These enable applications to be packaged inside virtual machines and allow multiple VMs to run on a single physical machine without interfering with each other. This provides increased utilization of resources and consolidation of server, space and data center costs.
Modern server virtualization technologies are transforming enterprise data centers including information technology (IT) hardware and software systems. By consolidating multiple physical bare-metal servers into fewer virtualized machines, enterprises are improving resource utilizations and reducing operational costs. Virtualization technologies provide administrators with the ability to manually set virtualization parameters. The virtualization parameters include a minimum amount of resource required for a VM (min, also referred to as a reservation), a maximum allowable resource (max, also called a limit) and its share of spare resource (shares, also referred as the weight in Xen schedulers).